Optical system with continuously variable magnification

ABSTRACT

An optical system for focusing the image of an object at continuously variable magnification on a surface substantially perpendicular to the optical axis and having two lens elements of positive refractive power axially movable relative to each other to vary the focal length of the system. A stop is fixed in the system relative to the image surface approximately in the midpoint of the range of movement of the front focal point of the two positive elements during relative movement of the same while the image is focused in the surface. The focal lengths of the two positive elements satisfy the relationship 0.6 f1&lt;f2&lt;3 f1 unless a negative lens element is located in front of the two positive lens elements in a fixed axial relationship of the rear focal point of the negative element to the aforementioned front focal point. In this arrangement, the focal lengths of the two positive elements need satisfy only the relationship 0.6 f1&lt;f2&lt;8 f1.

3 5 O 4 2 3 S R OR 3,57,35a

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[72] Inventors Yoshisada Hayarnizu; 2,003,881 6/ 1935 Grosset et a1350/(175TS) Satoru Sakamoto; Nobuo Yamashita, 3,020,806 2/ 1962Castrucci 350/96X Tokyo, Japan 3,433,559 3/1969 Vockenhuber et al.350/184X 1 9 1 5 968 FOREIGN PATENTS 1 pr. [45] Patented Apr. 27, 19711,425,097 12/1965 France 350/96 [73] Assignee Olympum Optical Co., Ltd.Primary Examiner-John K. Corbin Tokyo, Japan a Attorney-Kurt Kelman [32]Priority Apr. 20, 1967 [33] Japan [31] 42/24,833

ABSTRACT: An optical system for focusing the image of an object atcontinuously variable magnification on a surface [54] TINUOUSLYsubstantially perpendicular to the optical axis and having two 5 12Drawing as. lens elements of positive refractive power axially movableChum relative to each other to vary the focal length of the system. A[52] US. Cl 350/ 184, stop is fixed in the system relative to the imagesurface approx- 350/ 350/ imately in the midpoint of the range ofmovement of the front [51] Int. Cl. G02b 5/16, focal point of the twopositive elements during relative move- GOZb 13/221302?! l4 ment of thesame while the image is focused in the surface. [50] field of Search1550/96, Th fo al lengths of the two positive elements satisfy the rela-175 231 tionship 0.6 f, f, 3 f unless a negative lens element is locatedin front of the two positive lens elements in a fixed [56] Referencescued axial relationship of the rear focal point of the negative ele- EDSTATES PATENTS ment to the aforementioned front focal point. In thisarrange- 2,662,443 12/1953 Loeck 350/ 1 84X ment, the focal lengths ofthe two positive elements need 3,062,100 1 1/1962 Ludewig et a1.350/255X satisfy only the relationship 0.6f, f, 8f,.

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(est-H5 AM HAJR Him OPTICAL SYSTEM WITH CONTINUOUSLY VARIABLEMAGNIFICATION located in the aforementioned surface. The exit pupil ofthe objective is farther from the surface in a direction away from theobjective than the effective focal length of the objective.

The present invention relates to an optical system of continuouslyvariable magnification and more particularly to a system which can beused with an elongated image transmitting system having a very smalldiameter such as a fiber optical system.

The present invention is particularly useful in an endoscope which isprovided with a fiber optical system.

In an optical system of the prior an an objective lens is located infront of an elongated image transmitting system such as a fiber opticalsystem and the image transmitted to the rear end surface of the imagetransmitting optical system is viewed through an ocular. The focallength of the objective lens must be continuously varied, if it isdesired to continuously vary the size of the image of an object locatedat a fixed distance from the forward end surface of the imagetransmitting optical system as in a zooming lens system of a moviecamera. However, it is essential to reduce the number of the lenselements constituting the zooming lens system and to make themagnification varying mechanism thereof as small as possible, in orderto permit the system to be incorporated in an endoscope and the zoominglens system of the prior art cannot be incorporated in the endoscopebecause of the complicated construction and the large size of themorning lens system.

The present invention provides a novel and useful continuously variablemagnification optical system which can be used in an endoscope.

ln the attaching drawing:

FIG. I is a schematic view of the prior art fiber optical systemprovided with an objective lens;

FIG. 2 is a schematic view showing the principle of a well knowncontinuously variable magnification optical system;

FIGS. 3 and 4 are diagrams each showing the variation in the value fwith respect to the value d in the optical system shown in FIG. 2;

FIG. 5A and FIG. 5B shows an embodiment of the present invention inrespective positions;

FIG. 6A and FIG. 6B are views similar to FIG. 5 but showing anotherembodiment of the present invention;

FIG. 7A and FIG. 7B are schematic views of another optical system ofcontinuously variable magnification in two positions; and

FIG. 8A and FIG. 8B are views similar to FIG. 5A and FIG. 5B but showingstill another embodiment of the present invention.

Referring to FIG. I, an objective lens 0L is located in front of theforward end surface of an elongated image transmitting optical system Isuch as a fiber optical system at a distance therefrom. The image of anobject located in front of objective lens OL is focused on the forwardend surface of fiber optical system I. The image focused on the forwardend surface of fiber optical system I is transmitted through fiberoptical system I to the rear end surface thereof to form image Ithereon.

In a continuously variable magnification optical system Le. a variablefocal length optical system, the entire optical system is constituted byseveral lens elements or lens groups, and the focal length of theoptical system can be varied by varying the air gap(s) between theparticularly selected lens components. However, when the image receivingsurface which faces the objective lens is constituted by a fiber opticalsystem, it is necessary to direct the chief ray from the objective lensto the image receiving surface substantially at right angles. Therefore,the exit pupil of the objective lens must be located at a greaterdistance remote from the focal plane of the objective lens in comparisonwith the focal length of the entire optical system. When the focallength of the entire optical system is to be varied by varying thedistance(s) between the lens elements or lens groups, a mechanicallycomplicated system is required for shifting the position of the stopwhich satisfies the abovedescribed condition, that is, the stop which islocated at a position adjacent to the front focal point of a part of theentire optical system which is located behind said stop, along theoptical axis of the entire optical system. Therefore, it is desired toreduce the variation in the distance between the rear focal point andthe above-described stop. In other words, it is desired to reduce therange of shifting the position of the focal plane of the optical systemwhen the focal length of the optical system is varied.

As shown in FIG. 2, when two thin lenses having focal lengths f,, kfrespectively, are located at a distance df from each othe the focallength f of the optical system and the distance FF between the frontfocal point and the rear focal point of the entire optical system are:

Therefore, the focal length f of the optical system can be varied byvarying the distance d between the two thin lenses.

FIGS. 3 and 4 show the rate of variation inf with respect to k that isf/fo (f0 is the focal length of the entire optical system when d=0) andthe variation in the value FF with respect to k, respectively.

The range in which d can be varied is d 0, and the value of d is limitedby the fact that F, F cannot be located in the ranges in which said twolenses are located, respectively. Referring to FIGS. 3 and 4 with theabove facts being taken into consideration, the maximum value off can beobtagied when k=l while the minimum variation in the value of FF ismaintained. The value of k is not necessary to be precisely equal to 1.In consideration of the practically allowable movement of the exit pupilof the objective lens, the value of k should be between 0.6 and 3 toachieve the object of the present invention.

Therefore, it is possible to vary the focal length of the entire opticalsystem as described above by varying the relative distance between twopositive lens elements or lens groups each having substantially the samefocal length, while the variation in the distance between the stop solocated that the exit pupil of the entire optical system is positionedremote from the image receiving surface and the image receiving surfaceis limited to the minimum. As shown in FIG. 5, whose two parts 5A and 5Billustrate the principle of the present invention, the stop may belocated at substantially the midpoint of the range in which the value FFcan be varied by allowable changes in the value of d, and at a fixeddistance from the image receiving surface, while the exit pupil issufficiently remote from the image receiving surface, when the focallength of the entire optical system is varied.

As described above, the present invention provides an optical systemcapable of focusing images of the object on an image receiving surfaceat varying magnification while the distance between the stop of thesystem and the image receiving surface is kept substantially constant,said optical system having fewer lens elements and a simple mechanismfor varying the magnification.

In the above-described optical system, the upper and the lower limits ofthe focal length f of the entire system are automatically determined,when the focal length f, of one lens element is determined. However, inaccordance with one feature of the present invention, an afocal opticalsystem having the the operative range of the optical system to bewidened. And further. this arrangement has the advantage that the exitpupil can be located substantially at infinity.

FIG. 7A and FIG. 7B shows a known simple variable mag nification opticalsystem, in which a positive lens element is located behind a negativelens element or lens group, so that the image of an object is focused onthe image receiving surface located behind said positive lens element ata distance therefrom, and the distance between said two lens elementscan be varied thereby permitting the focal length of the entire opticalsystem to be varied.

In the case, the stop is located at the front focal point F of thepositive lens element L to satisfy the requirement that the exit pupilbe remote from the image receiving surface and said front focal point Fis always positioned between the negative lens element L, and positivelens element.

Asuming that the focal length of said negative lens element L, is f, andthat of said positive lens element L is f and that the rear focal pointof said negative lens element L is F, and the front and rear focalpoints of said positive lens element L are F F respectively, and thatthe distance between said lens elements d, then the focal length f onthe entire optical system is Therefore, assuming that the focal length fof the entire opif the focal length f should be changed to Bfo.

When the front focal point P must be positioned at the side of saidnegative lens element L, remote from the object, the minimum value ofIF, F is |f,| Therefore, when B is selected to be 0.5, for example, thensaid positive lens element L must be shifted by lfll from the initialcondition of \F, F 1 =Lf,| in the direction in which said distance d isincreased.

Therefore, when the focal length f, of the negative lens element L, issmall, the amount of said positive lens element L can be reduced toachieve the above-described effect. However, it is not desirable to makethe power of said negative lens element L, too high. because theaberration of the optical system is increased thereby and theconstruction of the optical system is complicated by the need forcorrecting the aberration of the optical system.

The focal plane of the entire optical system is shifted a distance whichis movement of the sum of the movement of the positive lens element orlens group L i.e.

and the value therefore the sum exceeds the movement of said positivelens element L thereby making the variation in the distance from theforward end of the entire optical system to the focal plane thereof toogreat. This is not practical.

In the present invention, as shown in FIG. 8, positive lens element L,shown in FIG. 7 is replaced by a positive lens system L comprising twopositive lens elements or lens groups Lu) and L ng Of respectivelengthsfuq andfuuy When the distance between said two positive lenselements or lens groups L and L is varied in such a manner that therelative position of the front focal point F H of said positive lenssystem L with respect to said negative lens element on lens group L, isfixed, then the focal length f of the entire optical system is magnifiedby B times when the focal length of said positive lens system L ismagnitude by 3 times, because the distance between the rear focal pointF of negative lens elementor lens group L and the front focal point F ofthe positive lens system L,, is kept constant although the focal lengthof said positive lens system L is varied.

Since the focal plane of the entire optical system is always shiftedrearwardly beyond the rear focal point F,, of the positive lens system Lthe upper limit of the distance d between the positive lens elements orlens groups L,,,,, and L which is allowable without the focal plane ofthe entire optical system entering the lens elements can be made greaterthan without the negative lens element or lens group L,, that is, theoptical system is constituted only by the positive lens system The focallength f of the entire optical system consisting of the negative lenselement or lens group L, and two positive lens elements or lens groupsL,,,,,, L which constitute positive lens system L is defined by theequation Therefore, the fact that the variation in the focal length fcan be made greater by the present invention permits the variation inthe focal length f of the entire optical system to be made greater. lnthis case, the power of negative lens element or lens group L, is notnecessarily high.

It is also possible to vary the upper and lower limits of the focallength f of the entire optical system by varying the position of saidpositive lens system L with respect to said negative lens element orlens group L,, while the ratio of variation in the focal length f of theentire optical system is kept constant. Since the value F, F is fixed,the front focal point F of said positive lens system L enters theposition of the negative lens element or lens group L, or is shiftedbeyond the negative lens element or lens group L,, however, this ispermitted in the optical system of the present invention. Therefore, thedistance between the forward positive lens element or lens group L,,,,,and negative lens element or lens group L, can be made extremely smallinsofar as the mechanical construction of the optical system permits.This is one of the characteristic features of the present invention.

Assuming now that the focal length of the negative lens element or lensgroup L, is pf,,,,, and the focal length f of the positive lens elementor lens group L is represented by kf,,,,, f,,,,, being the focal lengthof the positive lens element or lens group L and that the distancebetween the positive lens elements or lens groups L,,,,, and L isdf,,,,, then distance af from the front focal point F of the positivelens system L to the focal plane I is represented by the followingrelationship.

The condition that the focal plane I does not enter the range of thepositive lens element or lens group L when d is varied from 0 to k, isexpressed by the following relationship.

Since values of p and k are positive, the value of k must be greaterthan 1 if d is to be varied under conditions in which the front focalpoint F of the positive lens system L is prevented from entering therange of the positive lens element or lens group L,,,,,, at the sametime, the focal plane l' is prevented from entering the range of thepositive lens element or lens group L when the ratio of variation in thefocal length of the entire optical system may be held small, that is,when the value of d is not varied to approach the above upper limit, thelower limit of k may be practically 0.6 in consideration of theallowable shifting of the exit pupil.

If a in the equation (3) is a, and a,,, when d=o and d=k, respectively,then the value of k. by which the value a,,-a,, is reduced to theminimum, that is, the variation in the distance the from front focalpoint F of the positive lens system L to the focal plane I is reduced tothe minimum at the upper and lower limits of the variation in the focallength of the positive lens system L respectively, is determined interms of the value of p.

When the value of p is selected to be p=3, for example, then the valueof a,,a,, is reduced to a minimum when k L5. The value of k whichsatisfies the above condition approaches 1 when the value of p isincreased, while the above value of k increases to a value greater than1 when the value of p is decreased. However, if the value of k becomestoo great, the range over which the distance df j between two convexlens elements or lens groups L and L is varied in order to obtain thedesired ratio of variation in the focal length, i.e. the magnificationratio of the optical system, becomes too great. This is not desirable.Therefore, the upper limit of the value of k is practically 8.

As-described above, by providing a lens element or lens group havingnegative refractive power in front of the optical system in which twopositive lens elements or lens groups are arranged in series so as topermit the focal length of the optical system comprised of said twopositive lens elements or lens groups to be varied, the ratio ofvariation in the focal length can be increased and, at the same time,the power of said negative lens element or lens group is not necessarilyrequired to be high, thereby permitting the aberration of the opticalsystem to be advantageously compensated for.

[n the above description, each of the lens elements or lens groups isdescribed as being comprised of thin lenses, however, it is evident thatthick lenses or cemented compound lens elements can be as lenscomponents used in the objective of this invention.

The present invention is not to be limited to the embodiment asdescribed above and shown in the drawings, but it must be understoodthat the present invention includes the broad concept of the presentinvention.

We claim:

I. In an optical system having an axis and including objective means forfocusing an image of an object at continuously variable magnification ona surface substantially perpendicular to said axis, the objective meansincluding two lens components having positive refractive power andrespective focal lengths f f said components being axially movablerelative to each other, and the effective focal length of said objectivemeans being varied by the relative movement of said components, theimprovement which comprises:

a. a stop at a substantially fixed position relative to said surface andsubstantially in the midpoint of the range of movement of the frontfocal point of said two components during said relative movement whilesaid image is focused in said surface.

b. said stop and said components defining an exit pupil of saidobjective means spaced from said surface in a direction away from saidobjective means a distance greater than the effective focal length ofsaid components during said relative moment, said surface being a plane;and

c. a plurality of optical fibers jointly constituting an imagetransmitting system and an image receiving end surface of said imagetransmitting system, said end surface extending in said plane.

2. In a system as set forth in claim 1, said focal lengths f f,

satisfying the relationship 3. In a system as set forth in claim I, afurther lens component having negative refractive power coaxiallylocated in front of said two components, the rear focal point of saidfurther lens component being in a fixed position relative to said frontfocal point of said two components.

4. In a system as set forth in claim 1, said focal lengths 1], f satisfin the relationshi 5. In a system as set forth in claim I, said focalIengthsfl, f, being substantially equal.

1. In an optical system having an axis and including objective means forfocusing an image of an object at continuously variable magnification ona surface substantially perpendicular to said axis, the objective meansincluding two lens components having positive refractive power andrespective focal lengths f1, f2, said components being axially movablerelative to each other, and the effective focal length of said objectivemeans being varied by the relative movement of said components, theimprovement which comprises: a. a stop at a substantially fixed positionrelative to said surface and substantially in the midpoint of the rangeof movement of the front focal point of said two components during saidrelative movement while said image is focused in said surface. b. saidstop and said components defining an exit pupil of said objective meansspaced from said surface in a direction away from said objective means adistance greater than the effective focal length of said componentsduring said relative moment, said surface being a plane; and c. aplurality of optical fibers jointly constituting an image transmittingsystem and an image receiving end surface of said image transmittingsystem, said end surface extending in said plane.
 2. In a system as setforth in claim 1, said focal lengths f1, f2 satisfying the relationship0.6f1< f2< 3f1.
 3. In a system as set forth in claim 1, a further lenscomponent having negative refractive power coaxially located in front ofsaid two components, the rear focal point of said further lens componentbeing in a fixed position relative to said front focal point of said twocomponents.
 4. In a system as set forth in claim 1, said focal lengthsf1, f2 satisfying the relationship 0.6f1<f2<8f1.
 5. In a system as setforth in claim 1, said focal lengths f1, f2 being substantially equal.